BengeBoy 

Sort of by accident, I ended up going for a ride today with both a Cateye cyclometer and a Garmin Edge mounted on my bike. Cateye on the left, Garmin on the right. Having never done this before, I was, of course, curious about whether they would register the same data.

Though I set up both according to instructions (tire size, etc.), the Cateye consistently registered speeds about 0.5 mph faster than the Garmin.

I am compelled to understand this illogical inconsistency, and so I am asking for your help.

- If I go to the road forum, they will tell me that MPH doesn't matter and I should buy a PowerTap instead.

- If I go to the electronics forum, they will tell me to rewire one of the computers so they are consistent.

- The mechanics forum would tell me to re-read the manuals and re-adjust the computers.

- The training forum would tell me to throw away the Garmin, thereby increasing my average speed on all rides by 0.5 mph (a pretty big gain...).


However, I think only the 50+ forum has the wisdom to answer this question for me:

Why is the left hand side of my bike faster than the right?

City_Smasher

One of your legs is shorter than the other?

cranky old dude

NASCAR drivers have the same problem. Stop riding in circles.

forresterace

Of course, there are those that would say- "this is only natural, the right is always more regressive"

Try switching sides with each piece of equipment and see if suddenly the Garmin reads higher. Clearly this will prove the above supposition

kycycler

I think that you have the same inconsistency as I do. I tend to think that the Garmin is more accurate than the cyclometer. The cyclometer is set manually while the Garmin will pickup the wheel size automatically, unless you set the wheel size yourself. The Garmin registers the distance that the bike travels from the satellites it has picked up. This make me think the accuracy is higher than the cyclometer.

DnvrFox

Simple

0.5 multiplied by 100 (the proverbial CENTURY) = 50 - perfect for the 50+ forum!

The difference in your measurements is saying that you are a 50+'r

Houston

I do not believe that this question can be resolved without you giving the total miles reported by both electronic gizmos.

For concerns like these I tend to side with the roadies, except instead of watts, I am only concerned with my heart rate over time. I could give a flip about speed or power, my interest is my heartbeat.

Monoborracho

This is intuitively obvious to the most casual and disinterested observer.

You were going in a clockwise circle.

Road Fan

Is this teh P&R solution?

Allegheny Jet

Either you entered the wrong size for the tire when setting up the computer or it's the Coriolis effect.

Louis

I think it's due to magnetic lines of flux (heh-heh, he said flux).

Okay, I lied, I just wanted to say flux.

Jet Travis

This must be the 50+ Forum Someone actually uses "disinterested" in the proper context!

Road Fan

Seriously, I suggest what the mechanics would, except measure the wheel runout (distance covered by a wheel revolution) with your weight on the bike, it will change the reading on the wheel-based device. Then see if the small difference decreases.

It that doesn't fix it, rerun a distance test moving slowly, so the GPS has the maximum time to integrate out error. If it has an error averaging function, this will allow it to work best. Then adjust the 'fudge factor' to resolve the remaining error.

Ultimately there can be inconsistency between how the two computers, or rather the software, in the Cateye and the Garmin function - there's no SAE or international standard regulating their accuracy. Typically engineers would see these designs as non-critical, not safety critical. The accuracy by design would not be considered high precision.

Allegheny Jet

To thwart terrorist the government may be messing with the GPS system.

Louis said "flux", I didn't know it's allowable to write that word on the internets?

Retro Grouch 

Why that's easy.

Seattle is on the west coast so obviously the only direction you can ride is toward the east.
Since you are also in the northern hemisphere due to the curvature of the earth the left side of your bike has less distance to travel. Consequently it appears to be faster.

Next question please.

Hermes

I assume you have a cadence / speed sensor for the Garmin that measures mph as a function of wheel rotation. If not, then you get speed from GPS. GPS is not that accurate and hence the difference. If you have a Garmin speed sensor it may be a sampling issue. You may be sampling more data from one unit versus the other introducing error. This would be especially true if the actual wheel size was different from the size you inputted into the device.

byte_speed

I routinely run two speedometers, a Garmin Edge 305 for data collection and a cheap wired computer with numbers big enough for 50+ eyes to easily. So I think I can add a couple of thoughts.

Is the registered speed higher, or just the average speed? Depending on the autopause settings, the amount of stopped & slow time will vary between the two. This can make large differences in average speed if you are stopped often at lights, etc.

You did enter the wheel diameter manually on the Garmin? I always measure my roll out and enter it into both of my computers. They give very similar readings, except as noted above. The fact that one sensor is on the front wheel and the other on the back does lead to some differences, even though I measure the tires separately (because I use different pressure front/rear).

Shimagnolo

"A man with one watch knows exactly what time it is. A man with two watches is never sure."

maddmaxx 

You did account for the extra leap second when you performed the setup didn't you? How about the local gravity correction factor (for central Ct this is .00039 X 10 to the -13th). There are significant changes to the tire diameter as pressure increases or decreases and these changes can be greatly effected by the ambient temperature. For that matter, do you even know what the pressure of your tires was during the ride? (was that pressure guage or absolute). For a man who would run 2 computers on his bike concurrently, these are all matters of significant importance.




In the future I suggest that you leave one or the other devices at home, stop worrying and enjoy riding more. Your mind will be free of worry. Your bike will be lighter. Your air drag will be reduced.




One more thought, to reduce variables on the 2 computer setup, were you using the same wheel magnet to trigger both devices or was there yet one more inaccuracy introduced by excessive redundancy yet again.

jiminos

duh... the right side is lighter.

be well,

jim

tntyz

How is the Garmin set up? Is speed being measured by GPS or by the wheel sensor? If GPS, there can be small accuracy issues, but these should average out over a distance. If wheel sensor, is the diameter EXACTLY the same on both?

Road Fan

Despite my lecture about rollout, I'm kinda with Madmaxx - just use one, it ain't that critical to have them match, and why do you need the same answer twice anyway.

Another solution is to get a third device, record all three measurements, then assume the true answer is the one that best describes two of them (majority vote system).

These will never be high-precision measurements. Getting a loaded rollout is about as good as we mere mortals can get.

and here's the 50+ wisdom part: why do you need a distance measurement in the first place, and does the need justify high precision? If it's just curiousity or logging, I don't think you need a highly precise measurement. If the difference bothers you, only use one measuring device.

rainycamp

I'm curious not so much about speed, but distance. Did both units record the same number for the distance you rode?

zacster

You better bring the bike back to Elliott Bay and have them look at it. The space/time around the bike is definitely out of alignment. They have the tools there to take care of it.

maddmaxx 

Seriously, I would not expect accuracies of better than 2-5% at these prices and 10% would not shock me. I actually would expect to see differences between 2 identical units set up the same way. It only takes one or 2 missed or doubled pulses every now and then to make a noticable difference.

For reference, I believe that automobile speedometers are only spec'd to about 5% accuracy and even then, the analog dial units can vary wildly with temperature.